1. Field of the Invention
The present invention relates in general to electromagnetic reading of magnetically encoded cards, and more specifically to a sensor for detecting magnetized bits in regions in such cards.
2. Description of the Prior Art
Various readers and sensing devices for detecting magnetized bits in regions in magnetically encoded cards are known in the art as evidenced by U.S. Pat. Nos. 3,521,258 to Hurt, and to Rogers et al, 3,717,749.
The Hurt patent discloses an electromagnetic sensor for detecting magnetized bits, which sensor has two pairs of drive and sense windings. When the drive windings are pulsed with a current, a voltage is induced in each of the sense windings. The induced voltages oppose and cancel one another when the sensor is not in a close proximity to a magnetized bit. In contrast, when a magnetized bit is adjacent the sensor one pair of the drive and sense windings is positioned to be in a closer position to the bit than the other pair of windings so that the voltage induction between such one pair of windings is varied from a normal level. Such voltage variation is sufficiently great that cancellation of the voltages in the sense windings does not occur and thereby the presence of a magnetized bit is indicated.
In the Rogers et al patent, an electromagnetic sensor is disclosed that is formed of a high permeability core within a coil. Positioning a magnetized bit near one end of the core magnetizes the core to near saturation, and a current pulse through the coil at that time either fully saturates the core or drives the core away from saturation depending on the direction of the magnetizing force of the bit. The polarity of the bit is thereafter indicated by the difference in the pulse decay time resulting from increased or decreased coil inductance.
Although the above discussed sensors of Hurt and Rogers et al appear to provide satisfactory sensing of bits 0.125 inch (3 mm) or larger in diameter, there is an increasing need for sensors that will detect bits of much smaller diameters in order that considerably more information may be encoded in a credit card or the like. Furthermore, as more and more information is encoded in a card the importance of reliability of operation and simplicity of construction of a sensor for reading encoded cards is likewise increased.